Fos-related antigen (Fra-1), junB, and junD activate human involucrin promoter transcription by binding to proximal and distal AP1 sites to mediate phorbol ester effects on promoter activity.

Human involucrin (hINV) is a cornified envelope precursor that is specifically expressed in the suprabasal epidermal layers. We previously demonstrated that 2500 base pairs of the hINV gene upstream regulatory region confers differentiation appropriate regulation in transgenic mice. An analysis of the hINV gene sequence upstream of the transcription start site reveals five potential AP1 binding sites (AP1-1 to 5). Using reporter gene constructs in human keratinocytes, we show that the most distal (AP1-5) and most proximal (AP1-1) AP1 sites are essential for high level transcriptional activity. Simultaneous mutation of these sites reduces transcription by 80%. Gel supershift experiments indicate the interaction of these sites with Fra-1, junB, and junD. Involucrin mRNA levels increase 10-fold and promoter activity 5-11-fold when differentiation is induced by phorbol ester. Functional studies implicate AP1-1 and AP1-5 in mediating the phorbol ester-dependent increase in promoter activity. No involucrin promoter activity or involucrin mRNA was detected in 3T3 fibroblasts. We conclude that (i) two AP1 sites in the hINV promoter are important elements required for keratinocyte-specific expression, (ii) these AP1-1 sites mediate the phorbol ester-dependent increase in promoter activity, and (iii) Fra-1, junB, and junD may be important regulators of hINV expression in epidermis.